1. Technical Field
This invention relates to the control of relative humidity in substantially sealed machine enclosures.
2. Background Art
One type of machine to which the present invention may be applied is a magnetic disk file in which the disks are contained in a substantially sealed enclosure. Such a disk file is described in U.S. Pat. No. 4,054,931, assigned to the same assignee as the present application, which relates to a gas filtering arrangement for the file.
In the file described in that patent, the disk enclosure wall includes a bearing for supporting a drive shaft passing through the wall. A number of disks and a fan are mounted on the shaft within the enclosure. When the shaft rotates, the fan pumps air through a main filter, also within the enclosure, to clean the air therein. A breather filter, within the enclosure, is connected to the exterior by a breather aperture. The breather arrangement acts to equalize pressure as far as possible between the interior and exterior of the enclosure to reduce stresses caused by expansion and contraction of the air within the enclosure.
When the drive shaft rotates, a pressure distribution is created by rotation of the fan and disks such that a positive pressure is maintained adjacent to the bearing. This prevents air from entering the disk enclosure through the bearing and bringing with it contaminants such as grease droplets.
In a disk file such as that described above and also in other substantially sealed machine enclosures, high levels of humidity are often undesirable because of problems of condensation or corrosion of the enclosed components. In the case of disk files, the use of thin film deposition techniques to fabricate read/write heads and to provide the magnetic coating on the disks has been proposed. Such thin film head and disk components are more susceptible to corrosion than are the machined ferrite heads and particulate oxide coated disks which they replace.
Whenever a machine enclosure has a breather aperture or is deliberately or otherwise imperfectly sealed, there will be a small leakage of the surrounding atmosphere into the enclosure which will result in internal moisture concentrations equal to those of the surrounding atmosphere. If the running temperature of the machine is significantly above ambient temperatures the relative humidity within the enclosure may still be relatively low when the machine is in use. However, during power off periods the internal relative humidity is as high as the external humidity.
The prior art shows many examples of the use of desiccant materials to control humidity within various types of enclosures. In order to provide humidity control for the lifetime of a machine or enclosure, either a large quantity of desiccant is required or means must be provided to replace or recharge a smaller quantity of desiccant.
U.K. Pat. No. 1,072,528 discloses an air drying arrangement for the air space above the oil in an oil filled transformer. An external breather tube contains a desiccant charge for drying air flowing into the space as the oil contracts during power off. Heated oil from the transformer combined with air expelled through the breather tube by oil expansion after power on, is effective to recharge the desiccant. However, this action relies on a large change of volume of the enclosed space and the desiccant is external to and remote from this space.